These years have seen the orthogonal frequency division multiplexing (OFDM) employed for various radio communication standards such as the DAB, the DVB, and the IEEE 802.11a WLAN. The OFDM scheme has partially overlapped subcarrier spectra and is thus extremely vulnerable to carrier frequency offsets (CFO). Thus, the CFO has to be estimated and compensated for (Non-Patent Document 1). On the other hand, recent demands for inexpensive terminals have been met by employing the direct conversion receiver (DCR). However, the DCR can cause additional analog signal loss such as DC offset (DCO) or I/Q imbalance flicker noise. Only the most serious loss, i.e., the DCO, is herein taken into account, while other loss factors are assumed to be negligible.
The estimation of CFO in the OFDM system has been studied thoroughly because it is critical in maintaining the orthogonality of subcarriers. Some of literatures on those studies discuss the compensation for CFO and DCO in their simultaneous presence. The studies can be classified into two types. One is concerned with the method using pilot signals (Non-Patent Documents 3 and 4), and the other with the blind method. The approach using the blind method requires a thorough search in the target band based on calculations to estimate the CFO, leading to its complexity and the amount of calculations that is too much to be implemented. On the other hand, another approach which employs the pilot signal, except for the maximum likelihood method (ML), typically has a merit of reducing the complexity by making use of a special structure of the pilot signal. For this reason, we focus on an ordinary periodic pilot signal (PP). The mutual relation between PPs can provide a simple method for estimating the CFO in the absence of the DCO (Non-Patent Documents 1 and 2). On the other hand, the pilot signal in the presence of CFO and DCO has been studied in the literatures (Non-Patent Documents 3 and 4).
[Non-Patent Document 1] P. H. Moose, “A technique for orthogonal frequency division multiplexing frequency offset correction noise,” IEEE Trans. Commun., vol. 42, pp. 2908-2914, October 1994.
[Non-Patent Document 2] M. Morelli and U. Mengali, “An improved frequency offset estimator for OFDM applications,” IEEE Communi. Lett., vol. 3, pp. 75-77, March 1999.
[Non-Patent Document 2] C. K. Ho, S. Sun, and P. He, “Low complexity frequency offset estimation in the presence of DC offset,” in Proc. IEEE ICC '03, pp. 2051-2055, May 2003.
[Non-Patent Document 4] S. Marsili, “DC offset estimation in OFDM based WLAN application,” in Proc. IEEE GLOBECOM —04, December 2004.